1. Field of the Invention
The present invention relates in general to the field of electronic circuitry and, more specifically, to oscillator circuits.
2. Description of the Related Art
As microelectronic fabrication processes advance, there is an increasing demand for oscillator circuits that are capable of providing accurate, high-gain signal amplification while consuming minimal power. Power consumption of crystal oscillators is very important in many applications because of the need to implement “stop modes” and to perform “wake-ups” with minimal delays related to start-up time for the crystal oscillator. Low-power crystal oscillator designs, especially those employing capacitive coupling, require a large amount of resistance for internal biasing. Current methods for providing bias resistance require trade-offs between size, power consumption, and variability.
Current methods for implementing high-value resistors in crystal oscillators are generally too variable or too expensive. For example, a resistor providing 250 MΩ of resistance requires approximately 1.0 mm2 of silicon area for the highest density passive resistor currently available in a TSMC 0.25 micron process. These resistors could be made smaller; however, the decrease in resistor size comes at the expense of power consumption.
These resistors can be implemented with biased transistors operating in the linear region. However, the resistance provided by this approach is highly variable, with a possible variation of an order of magnitude for 100 mV change in Vds. This variation results in clipping and distortion on the oscillator signals, adding undesired harmonics and wasting power. Diode-connected transistors can also be used to provide bias resistance. However, this approach also results in undesired variability, clipping and excessive power consumption.
In view of the foregoing, it is apparent that there is a need for an improved low-power, high performance crystal oscillator circuit. In particular, there is a need for a high-performance crystal oscillator providing effective oscillator bias resistors with low power consumption and minimal substrate surface area.
Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.